This invention generally relates to display systems that form a two-dimensional image on a display surface and more particularly relates to a color display apparatus using spatial light modulators that are illuminated by light from both incoherent light sources and laser light sources.
Currently, promising solutions for digital cinema projection and home theater systems employ, as image forming devices, one of two types of spatial light modulators (SLMs): area SLMs and linear SLMs. An area spatial light modulator has a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate transmitted or reflected light from a light source. There are two salient types of area spatial light modulators that are conventionally employed for forming images in digital projection and printing apparatus: Digital Micro-mirror Devices (DMDs) and Liquid-Crystal Devices (LCDs).
Prototype projectors using one or more DMDs have been demonstrated. DMD devices are described in a number of patents, for example U.S. Pat. No. 4,441,791 by Hornbeck, issued Apr. 10, 1984, titled xe2x80x9cDeformable Mirror Light Modulator,xe2x80x9d U.S. Pat. No. 5,535,047 by Hornbeck, issued Jul. 9, 1996, titled xe2x80x9cActive Yoke Hidden Hinge Digital Micromirror Device,xe2x80x9d U.S. Pat. No. 5,600,383 by Hornbeck, issued Feb. 4, 1997, titled xe2x80x9cMulti-Level Deformable Mirror Device with Torsion Hinges Placed In A Layer Different From The Torsion Beam Layer,xe2x80x9d and U.S. Pat. No. 5,719,695 by Heimbuch, issued Feb. 17, 1998, titled xe2x80x9cSpatial Light Modulator With Superstructure Light Shield.xe2x80x9d Optical designs for projection apparatus employing DMDs are disclosed in U.S. Pat. No. 5,914,818 by Tejada et al., issued Jun. 22, 1999, titled xe2x80x9cOffset Projection Lens For Use With Reflective Spatial Light Modulators,xe2x80x9d U.S. Pat. No. 5,930,050 by Dewald, issued Jul. 27, 1999, titled xe2x80x9cAnamorphic Lens For Providing Wide-Screen Images Generated By A Spatial Light Modulator,xe2x80x9d U.S. Pat. No. 6,008,951 by Anderson, issued Dec. 28, 1999, titled xe2x80x9cOffset Projection Zoom Lens With Fixed Rear Group For Reflective Spatial Light Modulators,xe2x80x9d and U.S. Pat. No. 6,089,717 by Iwai, issued Jul. 18, 2000, titled xe2x80x9cProjector Apparatus.xe2x80x9d LCD apparatus are described, in part, in U.S. Pat. No. 5,570,213 by Ruiz et al., issued Oct. 29, 1996, titled xe2x80x9cLiquid Crystal Light Valve With Minimized Double Reflectionxe2x80x9d and U.S. Pat. No. 5,620,755 by Smith, Jr. et al., issued Apr. 15, 1997, titled xe2x80x9cInducing Tilted Perpendicular Alignment In Liquid Crystals.xe2x80x9d Conventionally, area SLMs are provided filtered source illumination from a lamp or other broadband source. LCDs may be of either the reflective type (Liquid-Crystal On Silicon, or LCOS) or the transmissive type.
Linear SLMs, which could also be considered as one-dimensional spatial light modulators, have some advantages over the two-dimensional LCD and DMD area spatial light modulators described above. Inherent performance advantages for linear modulator arrays include the capability for higher resolution, reduced cost, and simplified illumination optics. In addition, linear arrays are more suitable modulators for laser light than are their two-dimensional counterparts. Grating Light Valve (GLV) linear arrays, as described in U.S. Pat. No. 5,311,360 by Bloom et al., issued May 10, 1994, titled xe2x80x9cMethod And Apparatus For Modulating A Light Beamxe2x80x9d are one earlier type of linear modulator array that offers a workable solution for high-brightness imaging using laser sources, for example.
Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in commonly assigned U.S. Pat. No. 6,307,663, issued Oct. 23, 2001, titled xe2x80x9cSpatial Light Modulator With Conformal Grating Device.xe2x80x9d The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of ""663 has more recently become known as the conformal GEMS device, with GEMS standing for Grating ElectroMechanical System. The conformal GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of conformal GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated throughout most of an optical system, providing a high degree of system flexibility and allowing the use of lower cost optics. When used with laser sources, GEMS devices provide excellent brightness, speed, and contrast.
Commonly assigned U.S. Pat. No. 6,411,425, issued Jun. 25, 2002, titled xe2x80x9cElectromechanical Grating Display System With Spatially Separated Light Beamsxe2x80x9d and commonly assigned U.S. Pat. No. 6,476,848, issued Nov. 5, 2002, titled xe2x80x9cElectromechanical Grating Display System With Segmented Waveplate,xe2x80x9d (both to Kowarz et al.) disclose imaging systems employing GEMS devices in a number of printing and display embodiments. As with its GLV counterpart, a GEMS device modulates a single color and a single line of an for sequencing illumination and modulation data for each color to a single linear modulator or for combining separately modulated color images.
Among the recognized advantages of digital projection display employing spatial light modulators is an expanded color gamut, which allows displayed images to have improved color fidelity and appearance over images provided by conventional film-based or CRT-based projection systems. Color gamut is most readily visualized using the familiar tristimulus CIE color model developed by Commission Internationale de l""Eclairage (International Commission on Illumination), which shows the color space perceived by a standard human observer. FIG. 1a shows the CIE color model, which represents a visible gamut 200 as a familiar xe2x80x9chorseshoexe2x80x9d curve. Pure, saturated spectral colors are mapped to the xe2x80x9chorseshoexe2x80x9d shaped periphery of visible gamut 200. The interior of the xe2x80x9chorseshoexe2x80x9d then contains all mappings of mixtures of colors, including mixtures of pure colors with white, such as spectral red with added white, which becomes pink, for example. Within visible gamut 200, a device gamut 202 is typically represented by a triangle, with vertices approaching the curve of visible gamut 200. In FIG. 1a, device gamut 202, as drawn, approximates the familiar gamut for standard SMPTE (Society of Motion Picture and Television Engineers) phosphors, for example.
As is well known in the color projection arts, it is desirable for a display device to provide as much of visible gamut 200 as possible in order to faithfully represent the actual color of an image and to provide vivid colors. The component colors of a display, typically Red, Green, and Blue (RGB) define the vertices of the polygon for device gamut 202, thereby defining the area and shape of device gamut 202.
One basic strategy, then, to increase the size of device gamut 202 is to use light sources that are spectrally pure, or have at least a high degree of spectral purity. Lasers, due to their inherent spectral purity, are particularly advantaged for maximizing device gamut 202. Substantially monochromatic, laser sources effectively position vertices of device gamut 202 onto the periphery of visible gamut 200.
A number of digital projector designs have been proposed for taking advantage of the favorable spectral qualities of laser sources. For example, U.S. Pat. No. 6,183,092 by Troyer, issued Feb. 6, 2001, titled xe2x80x9cLaser Projection Apparatus With Liquid-Crystal Light Valves And Scanning Reading Beam,xe2x80x9d U.S. Pat. No. 6,426,781 by Lee, issued Jul. 30, 2002, titled xe2x80x9cLaser Video Projector,xe2x80x9d U.S. Pat. No. 6,435,682 by Kaelin et al., issued Aug. 20, 2002, titled xe2x80x9cLaser Imaging Using A Spatial Light Modulator,xe2x80x9d and U.S. Pat. No. 6,317,170 by Hwang et al., issued Nov. 13, 2001, titled xe2x80x9cLarge Screen Compact Image Projection apparatus Using A Hybrid Video Laser Color Mixerxe2x80x9d show just a few of the proposed approaches for digital projection using laser illumination sources. Designs such as those disclosed in the patents just listed take advantage of continuing advances in laser design and fabrication that provide increased power, improved lifetimes, and overall lower cost for laser illumination solutions.
However, in spite of significant advances, the lack of low-cost lasers in the visible blue spectrum remains a problem. Laser manufacturers have, as yet, been unable to provide blue lasers at reasonable cost in the power range needed for digital projection. In fact, the cost of lasers available in the visible blue spectrum can be as much as ten times the cost of green lasers at the needed power levels. To a somewhat lesser extent, the problem of cost and availability also affects red lasers in some power ranges, particularly those providing illumination for large screen projection. This problem, then, dramatically impacts the cost of a projection apparatus, making laser projection an unlikely near-term alternative for wide acceptance with projection systems.
While lasers provide light that is spectrally pure and therefore allow an enlarged color gamut, there are other characteristics of laser light that are less than favorable for digital projection. Notably, laser light is at least relatively coherent and can be highly coherent. As a result, speckle and other effects are a problem for digital projection devices using laser illumination. As is noted above, area spatial light modulators, particularly transmissive and reflective LCDs, although they perform well with conventional incoherent light sources, such as lamps and LEDs, are not well-suited for modulation of laser light. Instead, linear spatial light modulators, such as GLV and GEMS devices are preferred for use with laser illumination.
In general, incoherent light sources are not as constrained as are lasers with respect to blue wavelengths. For example, mercury arc lamps, widely available at the necessary power range for projection, radiate light in the visible blue range. In fact, the standard 436 nm line of mercury arc lamps provides a characteristic blue spectral component that is sharply defined. This allows a filter to be used to isolate and pass only this visible blue component. Thus, the mercury arc lamp can serve as an incoherent light source, providing light that is substantially spectrally pure, within the range of wavelengths that are not affordably achievable using lasers.
LEDs, while not as spectrally pure nor as bright as lasers, provide yet another possible low-cost incoherent illumination source for digital projection systems with small screens. LEDs can provide favorable solutions for some types of display apparatus, particularly since these devices are becoming more widely available at the needed wavelengths.
It is worthwhile to summarize these considerations for illumination sources in digital projection apparatus design:
(a) lasers, providing optimal color gamut and high brightness, work best with linear SLMs to provide high resolution, but may not currently be affordable at all needed wavelengths, particularly in the visible blue region;
(b) incoherent light sources, such as lamp and LED light sources, may not provide as broad a color gamut as lasers at comparable wavelengths. Incoherent light sources work best with area SLMs used at relatively lower resolution, and are available at wavelengths across the visible spectrum, where lasers are not currently affordable;
It is recognized in the digital projection apparatus design arts that contesting factors of different color gamut, SLM type, resolution, and wavelength represent a fork in the road. The decision to use either a laser-based illumination system or an illumination system using incoherent light sources, such as the more conventional lamp, dictates how the designer then proceeds subsequently in order to optimize apparatus performance and value.
Because light handling optics and image modulation methods differ significantly between laser-based illumination systems and other types, hybrid solutions do not appear to be attractive or even viable. As one type of hybrid solution, U.S. Patent Application Publication No. 2002/0154277 by Mukawa et al., published Oct. 24, 2002, titled xe2x80x9cImage Display Devicexe2x80x9d discloses an image display device in which a laser is added to a conventional lamp-based illumination system in order to supplement the available brightness at a specific wavelength. Notably, the system disclosed in the Mukawa et al. application uses the same type of spatial light modulator for modulation of both laser light and incoherent light from a lamp. However, this approach neither takes advantage of special properties of laser light for modulation nor compensates for imaging anomalies caused by coherent laser light when used with SLMs optimized for conventional light sources.
As is well known, a projected color image comprises a Red color image, a Green color image, and a Blue color image, superimposed, collectively termed an RGB image. The displayed color image has a certain resolution, typically expressed in terms of the number of horizontal and vertical pixels. In order to form the RGB image at the desired resolution, individual pixels within each red, green, and blue color plane are aligned to each other. Thus, using conventional approaches, each color plane, and each SLM corresponding to a color plane, have the same resolution.
It is known that the human eye, which combines the separate red, green, and blue pixels displayed in order to perceive composite colors, has different sensitivity to different colors. Green sensitivity, for example, is very high; the green color channel corresponds most closely to human perception of luminance. There is less sensitivity to red, and even less to blue. In fact, with particular respect to detail perception, the human eye is relatively insensitive to blue. That is, the actual resolution of the blue color channel is of relatively minor importance for perception of detail. For a displayed image, this effect can be shown dramatically by decreasing the resolution of only the blue image plane while maintaining the original resolution of the red and green color planes. Although the relative insensitivity of the human eye to blue for discerning detail is well-documented, conventional digital projection designs have failed to take advantage of this characteristic to ease design constraints, to benefit from expanded color gamut, and to provide, at the same time, lower cost projection devices.
The aforementioned needs and shortcomings are met by the present invention by providing an improved digital projector solution that employs different types of illumination and image-forming components on different color channels. With this solution in mind, the present invention provides a display apparatus for forming, on a display surface, a color image including a plurality of superimposed images, the display apparatus includes:
(a) a first color modulation channel for forming a first color two-dimensional image, comprising:
(a1) a laser light source for providing a first color source beam;
(a2) a linear spatial light modulator for modulating said first color source beam to provide a modulated light beam having a first color;
(a3) a scanning element for scanning said modulated light beam having said first color to form said first color two-dimensional image;
(b) a second color modulation channel for forming a second color two-dimensional image, comprising:
(b1) an incoherent light source for providing a second color source beam;
(b2) an area spatial light modulator for modulating said second color source beam to form a second color two-dimensional image; and
(c) at least one projection lens for projecting, toward the display surface, the color image comprising said first color two-dimensional image superimposed with said second color two-dimensional image.
Another aspect of the present invention provides a system for forming, on a display surface, a color image including a plurality of superimposed images, the system includes:
(a) a first color modulation channel for forming a first two-dimensional image, comprising:
(a1) means for providing a source laser beam;
(a2) means for modulating said source laser beam to provide at least one modulated light beam having a first color;
(a3) means for directing, toward a color combiner, a scanned line image beam including said at least one modulated light beam having said first color;
(b) a second color modulation channel for forming a second two-dimensional image, comprising:
(b1) means for providing a second color incoherent source beam;
(b2) means for modulating said second color incoherent source beam to provide a second color image beam to said color combiner;
(c) said color combiner combining at least said scanned line image beam and said second color image beam to form a superimposed color image beam;
(d) means for projecting said superimposed color image beam toward the display surface.
A third aspect of the present invention provides a method for forming, on a display surface, a color image as a plurality of superimposed images, including the steps of:
(a) forming a first color linear image beam including the steps of:
(a1) providing a first color laser source beam;
(a2) modulating said first color laser source beam to provide at least one diffracted light beam having a first color;
(b) forming a second color two-dimensional image beam including the steps of:
(b1) providing a second color source beam from an incoherent light source;
(b2) modulating said second color source beam;
(c) combining said first color linear image beam with said second color two-dimensional image beam to form a superimposed image beam; and
(d) projecting said superimposed image beam toward the display surface.
A feature of the present invention is the use of a combination of different types of illumination sources and different types of spatial light modulators within the same display apparatus.
It is an advantage of the present invention that it obviates the need for obtaining lasers at specific wavelengths in the visible region, particularly in the blue region. The present invention provides methods for using incoherent light sources as well as laser sources.
It is a further advantage of the present invention that it allows a display apparatus to use lasers on any number of color channels, where lasers are the most suitable and economical, for example, and to use other light sources where they can be most advantageous.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.